Single-crystal X-ray and neutron diffraction structure determination and inelastic neutron scattering study of the dihydrogen complex trans-[Ru(H 2)(H)(dppe) 2][BPh 4

Abstract

The structure of the complex trans-[Ru( η 2-H 2)(H)(dppe) 2][BPh 4]( 1),dppe = PPh 2CH 2CH 2PPh 2, has been determined by single-crystal X-ray diffraction at 123 K and neutron diffraction at 12 K. The core of the complex has a distorted octahedral geometry about ruthenium with the dihydrogen ligand trans to hydride and eclipsing a trans-PRuP axis that is bent away front the hydrogens with a PRuP angle of 167.9(4)°. The crystallographically determined H-H distance is 0.83(8) (X-ray) or 0.82(3) (neutron) Å. The latter value, where corrected for the shortening caused by the torsional libration of the H 2 ligand, increases to about 0.94 A. The long Ru(H 2) distance of 1.81(2) Å (neutron), compared to the terminal hydride to ruthenium distance of 1.64(2) A (neutron), is consistent with the lability of the dihydrogen ligand, which is partially lost from the crystal by treatment with vacuum. The analogous iron complex trans-[Fe(η) 2-H 2)(H)(dppe) 2[BPh 4]( 2) has similar features except that the FeH(H 2) distances are much shorter and the H 2 ligand is correspondingly less labile. An inelastic neutron scattering study of the powder of 1 at 5 K reveals two broad inelastic peaks flanking the elastic peak. With the assumption that the dihydrogen librates in a double-minimum potential, the barrier to dihydrogen reorientation is calculated to be 1.0 to 1.4 kcal mol −, depending upon which of the HH distances is used. This barrier is less than that for the iron analog, determined for its BF 4 salt, therefore suggesting that there may be less d π → σ ∗ backbonding in 1 than 2.